General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake 6.9 Energy Changes in Chemical Reactions Chapter 6 Chemical Reactions and Quantities 2013 Pearson Education, Inc. Lectures 2013 Pearson Education, Inc. Chapter 6, Section 9 2 Reaction Conditions There are three conditions required for a chemical reaction to occur. 1. Collision: The reactants must collide. 2. Orientation: The reactants must align properly to. break and form bonds. 3. Energy: The collision must provide the energy of. activation. 2013 Pearson Education, Inc. Chapter 6, Section 9 3 Activation Energy Activation energy is the amount of energy required to break the bonds between atoms of the reactants. If the energy of a collision is less than the activation energy, the molecules bounce apart without reacting. Many collisions occur, but only a few actually lead to the formation of product. 2013 Pearson Education, Inc. Chapter 6, Section 9 4 Activation Energy The activation energy is the energy needed to convert reacting molecules into products. 2013 Pearson Education, Inc. Chapter 6, Section 9 5 Heat of Reaction The heat of reaction is the amount of heat absorbed or released during a reaction. is the difference between the energy of breaking bonds in the reactants and forming bonds in the products. has the symbol H. H = H products H reactants 2013 Pearson Education, Inc. Chapter 6, Section 9 6 Exothermic Reactions In an exothermic reaction, the energy of the reactants is greater than that of the products and heat is released along with the products. the heat of reaction (H) value is written with a negative sign () indicating heat is released. 2013 Pearson Education, Inc. Chapter 6, Section 9 7 Endothermic Reactions In an endothermic reaction, the energy of the reactants is lower than that of the energy of the products. heat is absorbed and used to convert the reactants to products and written with a (+) sign. 2013 Pearson Education, Inc. Chapter 6, Section 9 8 Heat of Reaction Summary Reaction Energy Heat Sign of TypeChange in Reaction H Endothermic Heat absorbed Reactant side + Exothermic Heat released Product side 2013 Pearson Education, Inc. Chapter 6, Section 9 9 Learning Check Identify each of the following reactions as exothermic or endothermic. 2013 Pearson Education, Inc. Chapter 6, Section 9 10 Solution Identify each of the following reactions as exothermic or endothermic. Heat is a product. The reaction is exothermic. Heat is a reactant. The reaction is endothermic. Heat is a product. The reaction is exothermic. 2013 Pearson Education, Inc. Chapter 6, Section 9 11 Guide to Calculations Using Heat of Reaction ( H) 2013 Pearson Education, Inc. Chapter 6, Section 9 12 Calculating the Heat in a Reaction If 15.0 g of NO are produced according to the following reaction, how many kJ were absorbed? Step 1 State the given and needed quantities. Analyze the Problem. 2013 Pearson Education, Inc. Chapter 6, Section 9 13 Calculating the Heat in a Reaction If 15.0 g of NO are produced according to the following reaction, how many kJ were absorbed? Step 2 Write a plan using heat of reaction and any molar mass needed. g of NO molar moles of NO heat of kilojoules mass reaction molar mass Heat of reaction 2013 Pearson Education, Inc. Chapter 6, Section 9 14 Calculating the Heat in a Reaction If 15.0 g of NO are produced according to the following reaction, how many kJ were absorbed? Step 3 Write the conversion factors including heat of reaction. 2013 Pearson Education, Inc. Chapter 6, Section 9 15 Calculating the Heat in a Reaction If 15.0 g of NO are produced according to the following reaction, how many kJ were absorbed? Step 4 Set up the problem to calculate the heat. 2013 Pearson Education, Inc. Chapter 6, Section 9 16 Learning Check How many grams of O 2 reacted if 306 kJ were released in the following reaction? A g of O 2 B g of O 2 C g of O 2 2013 Pearson Education, Inc. Chapter 6, Section 9 17 Solution How many grams of O 2 reacted if 306 kJ were released in the following reaction? Step 1 State the given and needed quantities. Analyze the Problem. 2013 Pearson Education, Inc. Chapter 6, Section 9 18 Solution How many grams of O 2 reacted if 306 kJ were released in the following reaction? Step 2 Write a plan using heat of the reaction and any molar mass needed. kilojoules heat of moles of O 2 molar grams of O 2 molar mass Heat of reaction 2013 Pearson Education, Inc. Chapter 6, Section 9 19 Solution How many grams of O 2 reacted if 306 kJ were released in the following reaction? Step 3 Write the conversion factors including heat of reaction. 2013 Pearson Education, Inc. Chapter 6, Section 9 20 Solution How many grams of O 2 reacted if 306 kJ were released in the following reaction? Step 4 Set up the problem to calculate mass of O 2. Answer is A.